Contamination of groundwater by halogenated aliphatic contaminants, commonly known as chlorinated solvents is a frequent problem. In a survey of 945 water supplies, a group of researchers found trichloroethylene (TCE), tetrachloroethylene cis- and/or trans-1,2-dichloroethylene and 1,1-dichloroethylene to be the most frequently appearing compounds other than trihalomethanes. Westrick, J. J., et al., "The Ground Water Supply Survey", J. Amer. Water Works Assoc., 76(5):52-59 (1984).
Restoration of aquifers contaminated by these compounds have previously been based on the extraction of contaminated groundwater by pumping and subsequently treating the water at the surface. This method has proven to be effective but often entails great expense and also a risk of transferring the contaminants to another medium, such as the atmosphere. To circumvent these difficulties, in-situ treatment of contaminants has become a favorable alternative.
Various methods of in-situ restoration of chemically contaminated aquifers have been employed. For example, U.S. Pat. No. 3,846,290 discloses a process for the in-situ reclamation of aquifers contaminated by liquid fuels, such as hydrocarbons. The patent discloses a process of degrading hydrocarbon contaminants by first promoting proper growth conditions for a native population of microorganisms in the subsurface by simultaneous addition of oxygen and nutrients. The microorganisms then use the hydrocarbon contaminants as primary substrates for growth and are able to degrade those contaminants.
Biotransformation of halogenated aliphatic compounds in water has been demonstrated in laboratory studies using soil columns and mixed cultures. For example, Wilson, J. T. and Wilson, B. H. in "Biotransformation of Trichloroethylene in Soil", Applied and Environmental Microbiology, 29:242-243 (1985), disclose a laboratory transformation using microorganisms in an unsaturated soil column with an atmosphere of 0.6% natural gas and air. Fogel, M. M., et al. "Biodegradation of Chlorinated Ethenes by a Methane-Utilizing Mixed Culture", Applied and Environmental Microbiology, 51:720-724 (1986), discloses the biodegration of TCE, vinyl chloride, vinylidene chloride and cis- and trans-1,2-dichloroethylene by using mixed cultures of methaneoxidizing bacteria in the laboratory.
Henson, J. M. et al., "Metabolism of Chlorinated Aliphatic Hydrocarbons by a Mixed Bacteria Culture Growing on Methane", Abstract Q-97, Annual Meeting American Society of Microbiology, Atlanta, GA, p. 298 (1987) discloses the biodegradation of a range of both single and double carbon compounds by using mixed bacterial cultures in the laboratory.
U.S. patent application, Ser. No. 770,445, inventors Wilson, Jr., J. T. and Wilson, B. H., filed Aug. 29, 1985, discloses a method of transforming halogenated aliphatic hydrocarbons by treating contaminated water with microorganisms that metabolize gaseous lower molecular weight alkanes in the presence of oxygen.
In-situ biorestoration of aquifers contaminated by halogenated aliphatic compounds requires a different approach than that used to restore hydrocarbon-contaminated aquifers. Hydrocarbon contaminants can be used by microorganisms as primary growth substrates. However, in most cases halogenated aliphatic compounds cannot be used by microorganisms as primary growth substrates. The in-situ degradation of these compounds can be promoted by the biostimulation of a class of native microorganisms by introducing an appropriate primary substrate for growth (an electron donor) and an electron acceptor into the aquifer zone. This provides the microorganism with sufficient energy for growth to increase its population and maintain it at a level sufficient to bring about a significant rate of transformation. The microorganisms can then degrade the halogenated aliphatic compounds as secondary substrates.
TCE, a common groundwater contaminant, has been the focus of many efforts to use biotransformation to cleanse groundwater. For example, one method for biotransforming and thereby cleansing groundwater containing TCE is to inject water containing simultaneously dissolved methane and oxygen into the contaminated aquifer to stimulate the growth of methanotrophic bacteria to a population size that will be effective in transforming TCE. Two technical problems have resulted when this is done. First, the bacteria tend to grow closer to the point of injection than at locations further distant from it. This can result in clogging of the aquifer near the point of injection of the chemical and uneven distribution of biological activity in the aquifer. Second, higher concentrations of the primary substrate, which usually occur where the bacterial activity is highest, may compete for the enzymes used in transformation of the secondary substrate, thus reducing the rate of secondary substrate transformation.
The present invention circumvents these problems by providing a method for purifying an underground water supply containing halogenated aliphatic compounds by in-situ biodegradation of said compounds.
It is one object of the present invention to provide a method for enhancing the in-situ degradation of halogenated aliphatic compounds in groundwater.
It is a further object of the present invention to provide enhanced in-situ degradation of halogenated aliphatic compounds and other organic contaminants by the use of microorganisms that have the capability to transform halogenated aliphatic compounds by promoting biostimulation of a class of indigenous microorganisms through the introduction of an appropriate primary substrate for growth (an electron donor) and an electron acceptor into the treatment zone.
It is a further object of the present invention to provide a method for the biotransformation of contaminants by injecting into an aquifer zone, water containing the electron donor and acceptor, respectively, in a manner where they are separated at the point of addition, thereby preventing biofouling of the area close to the point of injection and permitting a more even distribution of transforming organisms throughout the aquifer zone.
It is still another object of the present invention to provide a method for biotransformation of groundwater contaminants wherein the primary substrate, or electron donor, does not compete for the enzymes used by the microorganisms in biotransformation of the secondary substrate, i.e. the halogenated aliphatic compounds, thereby providing an improved rate of secondary substrate transformation.
Other objects and advantages of the present invention will become apparent as the description thereof proceeds.